[ISSUE-95] Send replies from the right source IP address

Fixes #95

I didn't test this enough, but lets see what you have to say about it :)
I tried to keep it as simple as possible.

This has to be tested with IPv6. I guess that IPv6 is not supported yet (shouldn't be too hard to add IPv6 support though), but first lets make sure it doesn't break it.
V6 has
Cmsghdr (4 + 4 + 4) - http://golang.org/src/pkg/syscall/ztypes_solaris_amd64.go?s=2789:2852#L181
Inet6Pktinfo (16 + 4) - http://golang.org/src/pkg/syscall/ztypes_solaris_amd64.go?s=2854:2931#L187

This still adds to 32 bytes, but for some reason 32 bytes didn't work for me. I got a packet with 40 bytes OOB data.